1. Field of the Invention
The present invention is directed to an apparatus for the transmission of speech, and in particular to such an apparatus suitable for use in a hearing aid including an acoustic input transducer and an acoustic output transducer, and a threshold circuit interconnected therebetween.
2. Description of the Prior Art
German Patent No. 2,452,998 discloses a hearing aid including a broadband threshold switch, which acts on the output signal of the microphone (acoustic input transducer) so that the output signal is attenuated during pauses in speech, and is transmitted broadband in the presence of speech. This known circuit has the disadvantage that superimposed unwanted signals (for example, noise) are transmitted broadband in all frequency ranges when speech signals are present. As a result, the unwanted signals are audible in those ranges in which they are not covered by spectral components of the speech (for example, high-frequency unwanted signals remain clearly audible given low-frequency vowels, or the lower-frequency unwanted signals continue to be audible as well given higher-frequency sibilants).
A partially multi-channel circuit for suppressing unwanted signals, which functions without threshold switches, is disclosed in U.S. Pat. No. 4,185,168, and is also described in the article "Clinical Results Of Hearing Aid With Noise-Level-Controlled Selective Amplification," Hiroshi Ono et al, Audiology Vol. 22, 1983, Pages 494-515. A similar multi-channel hearing aid circuit is described in U.S. Pat. No. 4,508,940, which also operates without a threshold circuit.
German OS No. 30 27 953 discloses a hearing aid having a microphone, an amplifier stage with a modulator, and a receiver. A series of filter circuits which form frequency-selecting channels are connected in parallel to the output of the amplifier stage. A threshold circuit can be integrated into the evaluation circuits of the frequency-selecting channels. The output of the filter circuits is combined at a demodulator, from which the sum of the output signals is supplied to the receiver via a final amplifier, which may contain a sound diaphragm.